1. Technical Field
This invention generally relates to a process and apparatus for adsorbing and later desorbing contaminants from a fluid stream and, more particularly, to a filter utilizing a continuous sheet of activated carbon fabric which adsorbs contaminants from a contaminant-laden fluid stream, and which later desorbs the contaminants under controlled conditions. The filter is capable of periodic removal from use and is regenerable in a controlled environment at regeneration temperatures in excess of the in-use desorbtion temperatures.
2. Discussion
The use of activated carbon to adsorb contaminants, particularly hydrocarbons and other volatile organic compounds, is known in the art of filtration. One typical approach to filtration of hydrocarbons from a fluid stream is shown in FIG. 1 and involves forcing the contaminated fluid through a sacrificial bed of granulated activated carbon or particulate filters, referred to as pre-filters, and subsequently directing the fluid through a filter having a structure, typically either a stacked corrugated structure or a monolithic structure, made of a nonconductive inorganic substrate which is coated or an organic substrate containing activated carbon surfaces.
The filter is structured such that the substrate provides a honeycomb form, or a series of tubes which are closely spaced, to provide as much surface area as possible to contact the fluid as it flows through the filter. Since the activated carbon is carried on a substrate, the surface perpendicular to the fluid flow direction must be large, or the length of the filter in the flow direction must be long, in order to provide sufficient contact surface area with the activated carbon. The contact surface area is important because the contaminants in the fluid must contact the surface of the activated carbon in order to be adsorbed and removed from the fluid stream. If sufficient contact area is not provided, the contaminants will not be adsorbed and will therefore remain in the fluid stream. Once the fluid flows through the monolithic structure of the filter, the fluid is exhausted as presumably clean fluid.
In advanced systems the monolithic filter is positioned on a rotary device which provides in-use desorption of the filter. Other systems have been utilized where there are two or more parallel filter sets. In such a system, the fluid stream is switched from the first filter set to the second filter set when the first set is saturated. As the second set adsorbs the contaminants from the fluid stream the contaminants held in the first set are desorbed. The fluid stream is switched back to the first filter set when the second filter set is saturated. This type of parallel system is less effective than the rotary systems in many or most industrial applications and has fallen into disfavor.
In the rotary type system shown in FIG. 1, the filters are positioned around the rotary device such that a channel is created in a central portion of the filters. This channel acts as a clean exhaust channel through the center of the device. A portion of the rotary device, typically positioned opposite the fluid flow entry, is shielded from the incoming fluid flow and acts as a desorption area. The desorption area is intended to drive the adsorbed contaminants from the activated carbon surfaces of the filter.
Typically, hot air is forced through the honeycomb or tube passages of the monolithic structure when the filter is rotated to the desorption area. The hot air raises the temperature of the filter structure to between about 100.degree. C. and 180.degree. C. The raised temperature causes some of the adsorbed contaminants to become vaporized and desorbed from the activated carbon surfaces. The vapor phase contaminants enter the flow of the hot air stream which carries the contaminants as solvent laden desorption air to a secondary operation.
The secondary operation for the filter system of FIG. 1 is typically a thermal oxidizer or a condensation system. The thermal oxidizer heats the contaminants to a point where the molecular chain of the contaminants are broken apart and form non-hazardous molecules which can be safely discharged into the environment. The condensation system is used to cool the hot solvent laden air and collect the contaminants in liquid form as they condense from the air stream. The contaminants can then be processed for commercial use, can be further filtered and treated, or can be properly disposed.
One disadvantage of utilizing a hot air stream to desorb the contaminants from the filters is that the heat transfer properties of air are relatively inefficient. Another process for desorbing the contaminants from the filters has been the suggested use of electrical heating of the filter structure itself. This advantageously allows for a lower volume of air flow to carry the desorbed contaminants to the secondary operation.
Even though heating the filter structures to a temperature in the range of 100.degree. C. to 180.degree. C. liberates many of the contaminants from the filter, there is an ongoing problem with high boiling point contaminants which are not desorbed at these temperatures. High temperature boiling point contaminants are considered to include contaminants which have a boiling point above the in use heating temperature used in the present systems. As a result of leaving the high boiling point contaminants in the filter, the efficiency of the filter decreases over time. By leaving the high boiling point contaminants in the filter the effective surface area available to adsorb the contaminants as they flow through the filter is reduced and more contaminants will exit the filter and be exhausted into the environment.
A portion of this problem can be attributed to the materials used to form the structure of the filter. This problem is particularly prevalent in monolithic and corrugated structures which require the use of resins or binders. If the in use temperature of desorption were raised to a level which would desorb or pyrollize the high boiling point contaminants (i.e. 600.degree. C. or more), the binders used to form the structure of the filter would experience structural decomposition and would fail to properly support the honeycomb or tube formation required to allow fluid flow through the filter. It is further recognized that no heat source or method is presently used which can heat the filter to a temperature high enough, and in a short enough time period, to drive off all of the high boiling point contaminants during the in use desorption phase of the filtration system without structural decomposition.